Nitrogen is a fundamental building block for all life, forming the structure of proteins and genetic material. Although the atmosphere is nearly 78% nitrogen gas (N2), this form is largely unavailable to most organisms, requiring specialized processes to convert it into usable compounds. This conversion creates what scientists call “reactive nitrogen” (Nr), which includes compounds like nitrate and ammonium. For marine ecosystems, the supply of reactive nitrogen often acts as a limiting factor for the growth of photosynthetic organisms, such as phytoplankton, which form the base of the ocean food web. Understanding the pathways through which this nitrogen moves—from the atmosphere, rivers, and groundwater—is necessary for grasping how human activities are currently reshaping ocean chemistry.
Atmospheric Input Mechanisms
The atmosphere transfers reactive nitrogen from terrestrial and industrial sources to the ocean surface through two primary processes: wet and dry deposition. This flux is estimated at 46 to 67 Teragrams of nitrogen per year (Tg N yr-1) globally.
Wet deposition involves reactive nitrogen dissolving in water vapor and falling as rain, snow, or fog. The primary compounds are nitrate (NO3-) and ammonium (NH4+), originating from nitrogen oxides (NOx) produced by combustion and ammonia (NH3) from agriculture. When washed out, these compounds directly fertilize surface waters, especially in remote areas far from land-based runoff.
Dry deposition occurs when nitrogen gases and fine particulate matter settle directly onto the water’s surface without precipitation. This includes the settling of aerosols and the absorption of gases like nitric acid (HNO3) and ammonia (NH3). This constant settling contributes significantly to airborne nitrogen input, particularly near major source regions of pollution.
Fluvial Transport via Rivers and Runoff
River systems are the largest single conduit for moving terrestrial reactive nitrogen into the coastal ocean environment. Rivers act as the final collection point for nitrogen that has leaked from vast land areas, including agricultural fields, forests, and urban centers. Global estimates suggest that rivers transport approximately 40 to 66 Tg N yr-1 to coastal ecosystems.
The nitrogen transported by rivers exists in several chemical forms, reflecting diverse sources on land. The total amount delivered is closely tied to the intensity of human activity within its watershed.
The load is typically divided into:
- Dissolved inorganic nitrogen (around 40%), predominantly nitrate, which is highly bioavailable. This is often remnant synthetic fertilizer or atmospheric deposition not absorbed by terrestrial life.
- Particulate nitrogen (around 40%), bound in soil particles and organic matter eroded from the landscape.
- Dissolved organic nitrogen (approximately 20%), consisting of complex molecules from decaying material.
Watershed management practices, such as establishing riparian buffers or optimizing fertilizer application, can influence how much nitrogen reaches the river network. Once the nitrogen enters the flowing water, it is rapidly carried downstream, fueling excessive algal growth in estuaries and coastal zones. This riverine nitrogen contributes to environmental effects like eutrophication and the development of low-oxygen “dead zones.”
Subsurface Discharge through Groundwater
A slower, hidden pathway for nitrogen transport exists through the ground, known as Submarine Groundwater Discharge (SGD). This involves water traveling through coastal aquifers before discharging directly into the ocean or near-shore estuaries. This localized subsurface pathway is estimated to deliver about 4 Tg N yr-1 to near-shore ecosystems globally.
Groundwater flow acts as a delayed-release mechanism for contamination. Nitrogen from fertilizer or septic systems percolates slowly through soil layers over months or years. Pollution from past land use can thus continue to discharge into coastal waters long after the original source has been reduced.
The nitrogen in groundwater often consists of high concentrations of nitrate, leached through oxygenated upper soil layers. As this freshwater mixes with saline seawater in the coastal aquifer (the subterranean estuary), biogeochemical processes alter the nitrogen’s form. High concentrations of nitrate or ammonium can still reach the coastal environment, severely impacting near-shore areas and contributing to eutrophication and harmful algal blooms.
The Human Influence on Nitrogen Cycling
The scale of nitrogen transport via the atmosphere, rivers, and groundwater is directly traceable to a fundamental alteration of the global nitrogen cycle by human activity. Since the Industrial Revolution, human actions have effectively doubled the amount of reactive nitrogen cycling through the environment. This increase in supply is the core reason why these transport pathways are so impactful on marine ecosystems.
The most significant driver is the Haber-Bosch process, an industrial method developed in the early 20th century to synthesize ammonia from atmospheric nitrogen gas. This process is the foundation for the mass production of synthetic nitrogen fertilizers, enabling a vast increase in global food production. By converting inert nitrogen gas into bioavailable ammonia, humans created a massive source of reactive nitrogen continuously added to terrestrial landscapes.
Fossil fuel combustion is the other major human source, primarily through the release of nitrogen oxides (NOx) into the atmosphere. High temperatures generated during the burning of coal, oil, and gas (e.g., in power generation and vehicle engines) cause atmospheric nitrogen and oxygen to combine. These nitrogen oxides contribute to air pollution and are deposited as nitrate and other compounds onto both land and sea, feeding the atmospheric transport pathway.
The excess reactive nitrogen from these sources ultimately leaks into the environment, creating a cascade of effects across interconnected ecosystems. This surplus nitrogen eventually finds its way into the ocean via the riverine, atmospheric, and groundwater routes. This human-driven supply transforms natural transport mechanisms into major environmental concerns for coastal and open ocean health.